The invention relates to a transducer and apparatus which enables the force experienced by a simple force sensor to be converted into an electrical signal which is then capable of being used as a measure of the degree of force experienced by an object to which the sensor is attached. For example the invention may be used to realized a force sensitive washer which can be mounted between a retaining nut and an internal combustion engine cylinder head block in such a manner as to enable the pressure pulses caused by the engine to be detected and/or measured.
The electrical measurement of force is particularly useful in applications where for example, it is desirable to implement some degree of control over a process or operation that produces the force. One such example is an internal combustion engine in which the measurement of the force experienced at particular locations on the engine block or cylinder head, for example, can be used to determine the pressures exerted by the engine""s firing or combustion cycle. The exact timing of the point of maximum pressure is particularly useful information and can be gainfully employed in an engine management system such as might be realized by a controlling microcomputer. Hence a continuous electrical signal which is proportional to the engine pressures at all times is of particular use since this signal enables the precise timings of the various levels of engine pressure (and therefore engine cycle) to be accurately measured.
Force measurement is also useful in the control of fuel for internal combustion engines. In such engines, the air/fuel mixture ratios are precisely controlled in order to obtain optimum performance. Very often this strategy results in unwanted side effects such as the occurrence of engine-knock. A sensor capable of obtaining data representative of engine pressures can be used in this context to enable the early detection of the onset of engine-knock and hence facilitate the so-called xe2x80x9clean burnxe2x80x9d mode of operation of the engine that is popular among engine manufacturers.
Techniques for the electrical measurement of force, particularly with regard to load sensing are known. For example, methods for measuring force utilizing piezoelectric materials such as quartz or lead zirconium titanate are well established. Another popular technique for measuring force involves using the piezoresistive effect exhibited by certain materials and the use of electrical resistance strain gauges in this context is known.
Of the piezoelectric devices, most of the devices employed in the prior art are expensive due to the need for complex manufacture to render them suitable for mechanical mounting. In addition the associated electrical circuits needed to measure their output signal, which takes the form of released charge, are also complex and expensive. Further disadvantages of the use of piezoelectric devices are their poor response at low frequencies and their high susceptibility to the unwanted effects of temperature variations.
Piezoresistive devices generally use comparatively inexpensive electrical circuits to measure the changes in resistance which result from the strain experienced due to applied force. A major problem in many applications, however, is the difficulty of mounting resistive strain gauges in a suitable mechanical arrangement. This latter consideration stems from the fact that it is often impossible to locate a resistive strain gauge at a point where it will experience sufficient strain to enable an accurate measurement to be made. Also the use of adhesives which is often necessary in the mounting of resistive strain gauges, makes them particularly susceptible to the problem of creep between the gauge and the supporting structure. In particular, metal foil resistive strain gauges, which are popular sensing elements in commercial force sensors, suffer from a relatively low sensitivity.
The characteristics of piezoresistive devices used in the measurement of force can be significantly improved if the force sensing elements are fabricated as thick film resistors. As described, for example in GB-A-2310288, it is possible to print force-sensing resistors onto an electrically insulating substrate and then sandwich them between this substrate and an electrically insulating material in such a way as to form a force sensing washer. By measuring the change in resistance of the force sensing resistors it is then possible to determine the degree of force applied to the washer.
According to the invention a force sensing transducer comprises; an electrically insulative support substrate, a force sensitive resistance comprising a thick film resistor formed as a plurality of overlapped layers of thick film material printed and fired onto the support substrate, a reference resistance located on the support substrate, and a force transmitting member overlying the force sensitive resistance and arranged to transmit force to the force sensitive resistance and not to the reference resistance, whereby the change in resistance of the force sensitive resistance relative to the reference resistance is indicative of the magnitude of a force applied between the support substrate and the force transmitting member.
In order to measure the change in resistance of the force sensitive resistors it is desirable to connect them to an electrical circuit such as a Wheatstone""s bridge for example. In this arrangement the change in resistance of the force sensitive resistors (used for force sensing) relative to fixed value (reference) resistors included in the bridge results in an out-of-balance condition in the bridge which produces an output voltage change proportional to the degree of applied force.
The choice of the fixed value resistors largely determines the characteristics of the resulting force sensor. For example, a sensor with good thermal stability can be achieved if the fixed value resistors and the force sensing resistors share the same temperature characteristic and are subjected to the same thermal excursions. This can be achieved if the fixed value resistors are fabricated identically to the force sensing resistors and are located on the same substrate material but at a point of constant force regardless of the degree of force applied to the force sensing resistors.
Similarly, for a full Wheatstone""s bridge configuration the optimum output signal change for any given applied force will be obtained when the resistance values of the fixed resistors are identical to the resistance values of the force sensing resistors with zero applied force. This condition can be conveniently achieved when for example, the force sensing resistors and the fixed value resistors are simultaneously fabricated as thick film resistors. In this way, both types of resistor can be simultaneously printed with the same thick film material and to a common geometry and, in particular, thickness.
The present invention also makes it possible to fabricate a force sensitive device in which the fixed value resistors and the force sensitive resistors are co-located onto a common substrate such that the force sensitive resistors are subjected to the applied force whilst the fixed value resistors are not. In this way it becomes possible to fabricate both the force sensing and fixed value resistors as matched devices sharing the same thermal environment